Formally, the schemes (REF ) - (REF ) represent a system of linear algebraic equations. Nevertheless, each \(i\) -th equation in (REF ) - (REF ) contains at most one neighbor value, either \(\phi _{i-1}^{n+1}\)  or \(\phi _{i+1}^{n+1}\)  (or none). Such linear equations can be solved
using one forward and backward substitution as known for fast sweeping methods [1]}.
